The orientation of crystals in a single Kevlar(R) 49 fibre has been measured by X-ray diffraction using a microcompressive device and the Cornell High Energy Synchrotron Source (CHESS). The data show that the orientation with respect to the fibre axis decreases with increasing axial compressive strain. The birefringence has been measured on the same sample with a polarizing optical microscope and a tilting compensator. The results show that the birefringence decreases with increasing axial compressive strain. This also is consistent with a decreasing crystal orientation with increasing strain. Both effects are slightly nonlinear and a graph of the full width at half maximum of the crystal orientation versus birefringence is nearly linear. The decreasing crystal orientation in compression is opposite to the increasing orientation with increasing tensile strain. The tensile effect increases the fibre modulus. The compressive effect, on the other hand, should decrease the modulus. The former has a limiting effect, but the latter could have the opposite and might contribute to compressive failure.